During simple single-joint movements, it is reasonable to assume what type of muscle contraction is occurring. During a basic elbow curl in the upright position, as the elbow flexes and the weight come up, the muscle is shortening so this would be called concentric. As the weight is lowered and the elbow extends, the muscle is lengthening and the contraction would be considered eccentric. If at any time as the weight is moving up or down, the movement stops there would cease to be a length change and it would be an isometric contraction.
From a muscle torque perspective there are 3 conditions. If the elbow is flexing, the torque generated by the muscles(s) is greater than the torque being created by gravity on the forearm and weight. If the weight is being lowered then the muscle torque is less than what gravity is creating. If the movement stops, then the internal torque (muscle) and the external torque (gravity) are equal.
But a basic question remains: does the muscle function differently, or is it just a case of the balance of torques? When science created the ability to pair movement analysis with electromyography (EMG), it was determined that eccentric contractions were more efficient because there was less EMG lowering a weight with gravity than raising it against gravity. This added to the idea that eccentric contractions were different than concentric. Then studies showed that delayed-onset muscle soreness was worse with a lot of eccentric (decelerating) contractions. These two facts (and others) have created a mindset of “different types” of contractions.
An alternative perspective could be that the muscle doesn’t know any difference in terms of the contraction. The difference comes from the length changes in muscle created by the internal and external torques. The muscles create enough torque to raise the weight, and then create less torque that allows gravity to lower the arm and weight. But, the muscles do not create an eccentric or concentric. The muscle creates tension that produces torque to accomplish the task. The reader might be asking: What’s the point? Well, if the contraction is just that, tension development, then our approach to movement changes. Our goal is to create movements. The movements result from the interplay of torques. The movements determine if the muscle is lengthening, shortening, or unchanged.
An earlier blog on “econcentric” muscle function explained how muscles could be shortening and lengthening at the same time. The term “econcentric” was used to represent the muscle activity during three-dimensional movements. At a tri-axial joint (hip) a muscle could be lengthening in each of the three planes or shortening in each of the three planes. Those two situations would “fit” the concentric – eccentric definitions. But if the joint motion simultaneously creates both shortening and lengthening in different planes, then there are six other possible combinations that would be “econcentric”. Then, there is the possibility of no movement which would suggest that the muscle is contracting, but not changing length (isometric). But there could be movement in multiple planes without a change in length if the lengthening in one plane(s) equals the shortening in the other plane(s). Would we call this a dynamic isometric? If we look at muscles that cross more than one joint, then the complexity increases and the number of combinations multiplies. Our simplistic definitions of muscle contractions are no longer instructive.
It doesn’t matter what we call the contraction. What matters is that we recognize the complexity and let it guide us in rehabilitation, performance training, and injury prevention. We must design programs involving movement at multiple joints moving in three planes. This will allow the muscles to learn to create the proper level of tension (combined with other muscles and the forces of gravity and passive stretch) to accomplish the desired task.